Barra – the Physical Background

Barra Barra - the Physical Background Noel Fojut AS WITH all islands, the physical facts of Barra's location, geology and climate have a profound influence upon the nature of life for inhabitants and visitors. Even today the constraints of time, tide and weather are no respecters of timetables, while weather variations within a single day can give the lie to the changing seasons. How much more so in, centuries past, must the physical geography of Barra have determined human options, closing off opportunities with one hand while opening up connections with the other. Determinism may be an unpopular concept in modern science, yet anyone spending a winter on Barra might think again. Barra's story begins in an unimaginably distant geological past. An area oflighter crustal rocks, a 'craton' as geologists of 3000 million years later would term it, formed the basis for an early continent (Fettes et al 1992: passim)1. This area, of unknown extent but including much of what is today the Western Isles, rode high above the heavier rocks of the ocean floor. Despite endless crustal movements, continents spreading and being destroyed, oceans opening and closing, these rocks remained unabsorbed. Most of the upper crust of the Earth from that early date has long been re-cycled, drawn down into the lower reaches, melted and re-cycled. But not the rocks of Barra. Despite intense pressure and folding, sometimes as much as 25 km below the surface at the root of an ancient mountain chain comparable to the Alps or the Andes, they survived, to form what today we call the Lewisian gneiss. The gneiss, with its distinctive grey hues mottled with patches of granite, was already ancient when crustal movements created a huge fault. Running the full length of the east coast of the Long Island, this is the Outer Hebrides Thrust, Fettes et al 1992 covers the Outer Hebrides. It is intensely detailed, requiring good geological knowledge to read with any degree of profit. Boyd and Boyd 1996: 11-29 provides a good non technical geological summary for the whole of the Hebrides. 12 Barra created around 1700 million years ago. Rocks from the east rode up westwards over more recent rocks, in a manner analogous to the better-known Moine Thrust of the North West Highlands. Recent research has suggested that at times the Thrust later acted as a normal fault, with younger rocks displaced downwards to the east, towards the Minch. Research work continues - a fault line so ancient will have had more than one incarnation, and with movements in both normal and reversed directions during its long life. There are still occasional movements along the fault today. N 1 D TF OHT 0 O...._....__.....__.___.__, 5 km Fig I Geological features affecting the modern landscape of Barra: OHT =Outer Hebrides Thrust; TF =tension faults. Key locations mentioned in text: b =Beul a'Bhealaich; c = Castlebay; f = Fuiay; h = Heaval; k = Kisimul Castle; o = Orosay; t = Ben Tangaval; I = Cleat; 2 = Traigh Varlish. Noel Fojut - the Physical Background 13 The Outer Hebrides Thrust, which like most thrusts is composed of several sub-parallel lines of movement, is clearly visible in the landscape of present-day Barra. Crushing and sliding movements along a broad plane transformed the rocks on each side of the fault, creating harder rocks where heat was generated by the movements and softer rocks where crushing was predominant. The large slab of rocks running uphill behind Castlebay toward the summit of Heaval is composed of rocks partially liquidised and reset. Kisimul Castle sits directly on the fault, its rock made a little harder than the surroundings. The gaps between the rocky islands at the northeast end of Vatersay, so clearly visible from Castlebay, correspond to rocks crushed by fault movements. A band of altered rock along the fault line runs very visibly across Fuiay, off the eastern entrance to Northbay. On the west side of Barra a secondary fault, apparently of the same age, curves around the west side of Ben Tangaval. Then, for a little over a mere 1000 million years, there is no record of geological change: for most of this time, no doubt, modern Barra lay deep below crumbling mountains, slowly rising as erosion removed the vast overburden, which was washed off in great sheets of sands and gravels, some of which went to form the Torridonian sedimentary rocks of the western seaboard of the Scottish mainland. About 400 million years ago, crustal mo.vements reawakened the old continental core of the Outer Hebrides once more. The great fault was reactivated, and as mountains built up over what was to become the Scottish mainland the crust was stretched slightly, creating a series of faults that now run more or less east to west across Barra. These became lines of weakness to be exploited by later erosion, and today are dominant landscape features. The gaps between Barra and Vatersay, the dip running from Borve up the valley and under Beul a Bhealaich across to Earsary and, running north-west to south-east, the fault which runs across the inner end of Ardmhor and forms the seaward portion of Loch Obe, are all examples of Caledonian fault lines. Meanwhile the Minch, formed by down-faulting along a north-south line running offshore just to the east of the Outer Hebrides Thrust, had become what it remains today - a great fault-defined trough into which sediments were washed from surrounding shores. Over the millennia, Barra may have been submerged from time to time, receiving a capping of sedimentary rocks, but any such deposits have long since eroded. The general pattern, of the Outer Hebrides riding high on the western 14 Barra edge of an active geological zone, continued until relatively recent times. 70 million years ago the Atlantic began to open, accompanied by a great burst of volcanic activity, which threw up the great gabbro masses of the Rum and Skye Cuillin and the complex rocks of Mull and Ardnamurchan. The opening of the Atlantic split Greenland and Labrador apart from Western Scotland. The landscapes of most of the Inner Hebrides were dramatically transformed at this time, with the creation of distinctive volcanic features, especially in Skye, Mull and Rum. But on Barra, only a few dykes of intruded igneous rock mark the violence experienced a few kilometres to the east. Examples may be seen along the northwest shore of Vatersay, around Traigh Varlish, and on the headland leading towards Orosay at Eoligarry. These dykes are outlying products of the volcanic centre which today is marked by Ben More on Mull. On Barra, the slow erosion of the Lewisian gneiss continued. By this time dinosaurs were abroad, and the landmass that was later to become Barra lay 30 degrees south of the Equator, enjoying a sub-tropical climate. The generally rounded contours of the island's overall shape, with deep-weathered pockets, are a product of the millions of years the ancestral Barra spent drifting slowly northwards through tropical zones. The broad outlines of Barra's landscape were determined long ago. But one last episode was to come. The changing shore lines of the widening Atlantic, perhaps combined with changes elsewhere and maybe global climate changes, led to a period when ocean circulation failed to redistribute warmth northwards, and the Ice Age was born. Like much of Scotland, Barra would have been affected by the four main cold phases and many smaller oscillations. At times it was covered completely by ice moving west from the Scottish mainland, while at others it probably supported its own small ice cap, or a southern lobe of a Long Island ice cap. But it has to be said that the old, hard rocks were little affected by such trifling matters as several hundred metres of ice grinding over them, and there are no classic glacial erosion features on the island. Some shallow clays and gravels were laid down; a few rock faces steepened, and much of the soil swept away. But Barra was already too gently rounded for the ice to make much difference (Hall 1996:5-11). Offshore, matters were very different. Deep submarine troughs occur along the east side of the island, reaching up to 250m deep at the south end of the Barra Isles. These have been attributed to glacial erosion, but it is hard to fit into any of the terrestrial evidence, even allowing for softer sediments and weaknesses along the Minch Fault (Sissons 1967:52). On the western edge of Noel Fojut - the Physical Background 15 the continental shelf, in the Atlantic beyond Barra, vast quantities of glacially eroded debris carried out by the glaciers from the west of Scotland piled up, subsequently collapsing in vast underwater avalanches down the slope into the depths to create the Barra Fan. (Holmes 1997:92). This is the southernmost of a series of such continental slope deposits which includes those of Norway responsible for the huge, and increasingly well-documented Storegga Slide of about 6000 BC, which created a tsunami responsible for depositing a layer of sand on coastal sites along the eastern Scottish littoral (Smith 2002:468). tN ;1;,:/ 0 50km /; /," ./· Fig. 2 Hebrides and West Highland coast, showing approximate coastline at lowest sea level immediately after last glaciation. 16 Barra Barra has important claims to the interest of geomorphologists studying relatively recent phenomena. At Cleat2 a beach of small gravel, plastered on the foot of the cliff at the east end of the bay, appears to date from the period before the last glacial episode. It looks as if the beach was frozen in situ and then buried beneath glacial till from the last ice-cover.

Load more